Hereinafter, a conventional apparatus for producing a silicon single crystal by the Czochralski method will be explained by exemplifying growing a silicon single crystal.
FIG. 4 shows a schematic sectional view of an example of the conventional apparatus for producing a silicon single crystal.
In the apparatus for producing a single crystal 101 used for producing a silicon single crystal by the CZ method, crucibles 106 and 107, and a heater 108 are generally arranged in a main chamber 102 where the single crystal 104 is grown, the crucibles which accommodate a raw material melt 105 and can be moved upwardly and downwardly, the heater which is arranged so as to surround the crucibles 106 and 107. A pulling chamber 103 for accommodating and taking out the grown single crystal is continuously provided above the main chamber 102. The crucibles 106 and 107 are supported with a crucible rotating shaft 118 that can be rotated and moved upwardly and downwardly by a rotation drive mechanism (not shown) installed at a lower portion of the apparatus for producing a single crystal 101.
A heat insulating member 109 for preventing the main chamber 102 from being directly exposed to heat from the heater 108 is provided outside the heater 108 so as to surround a perimeter of the heater.
For the purpose of discharging impurities generated in the furnace out of the furnace etc., an inert gas such as argon gas is introduced into the chambers from a gas inlet 111 provided at an upper portion of the pulling chamber 103, passes through the single crystal 104 during pulling and a surface of the raw material melt 105 to circulate inside the chambers, and is discharged from a gas outlet 110. There is provided a gas flow-guide cylinder 114 for guiding the inert gas so as to flow downwardly near the crystal from above the melt.
The cooling cylinder 112 extends at least from a ceiling of the main chamber 102 toward the surface of the raw material melt 105 so as to surround the single crystal 104 during pulling. A cooling medium is introduced into the cooling cylinder 112 from a cooling medium inlet 113, circulates through the inside of the cooling cylinder 112 to forcibly cool the cooling cylinder 112, and then is discharged outside.
In the case of producing the single crystal by using the apparatus for growing a single crystal 101 as described above, a seed crystal 116 is immersed in the raw material melt 105 and carefully pulled upwardly with being rotated to grow a rod-shaped single crystal, while the crucibles 106 and 107 are moved upwardly according to the growth of the crystal so that the melt surface is always maintained at a constant height in order to obtain a desired diameter and desired crystal quality.
When the single crystal is grown, the seed crystal 116 attached to a seed holder 117 is immersed in the raw material melt 105, and then a wire 115 is carefully wound up with rotating the seed crystal 116 in a desired direction with a pulling mechanism (not shown) to grow the single crystal 104 at an end portion of the seed crystal 116. Here, in order to eliminate dislocations generated when the seed crystal 116 is brought into contact with the melt, the crystal is once made thin to a diameter of about 3 to 5 mm in an early stage of the growth, and then the diameter is increased up to a desired diameter after the dislocations are eliminated so as to grow the single crystal 104 having desired quality.
In this case, a pulling rate for a portion having a constant diameter of the single crystal 104, although depending on the diameter of the single crystal to be pulled, is usually extremely slow, for example, about 0.4 to 2.0 mm/min. If it is pulled fast by constraint, the single crystal during the growth is deformed and consequently a cylindrical product having a constant diameter can be no longer obtained, or there arise problems that slip dislocations are generated in the single crystal 104, the single crystal 104 cannot be a product by being detached from the melt and the like. Thus, increasing the growth rate of the single crystal is limited.
However, for the purpose of improving productivity and reducing cost in the foregoing production of single crystal 104 by the CZ method, the increase in the growth rate of the single crystal 104 is one of considerable means, and accordingly various improvements have hitherto been made in order to achieve the increase in the growth rate of the single crystal 104.
It is known that the growth rate of the single crystal 104 is determined by heat balance of the single crystal 104 during the growth and can be increased by efficiently removing the heat emitted from a surface of the single crystal. In the case, an enhancement of a cooling effect on the single crystal 104 enables the single crystal to be further efficiently produced. Furthermore, it is known that a cooling speed of the single crystal 104 varies crystal quality. For example, Grown-in defects formed in the silicon single crystal during the growth of the single crystal can be controlled by a ratio of the pulling rate (the growth rate) to a temperature gradient in the crystal, and a defect-free single crystal can be grown by the control (See Japanese Patent Laid-open (Kokai) No. H11-157996). Thus, the enhancement of the cooling effect on the single crystal during the growth is important for producing the defect-free single crystal and for improvement productivity by increasing the growth rate of the single crystal.
In order to efficiently cool the single crystal 104 in the CZ method, effective is a method of absorbing radiant heat from the single crystal 104 into an object that is forcibly cooled such as the chamber without directly exposing the crystal to radiant heat from the heater 108. Screen structure is apparatus structure that can realize this (See Japanese Patent Publication No. S57-40119). However, in this structure, screen shape for avoiding contact due to the upward movement of the crucible needs a smaller diameter of a screen upper portion. The screen structure therefore has fault such that it is difficult to cool the crystal.
In addition, there is also a problem such that the cooling effect on the single crystal cannot be utilized, the effect which is brought about by flowing the inert gas during crystal pulling to prevent contamination due to an oxidizing gas.
In view of this, there is proposed structure that comprises a gas flow-guide cylinder for guiding the inert gas and a heat shielding ring for shielding the direct radiant heat from the heater and the raw material melt to the gas flow-guide cylinder (See Japanese Patent Laid-open (Kokai) No. S64-65086). In this method, the cooling effect of the inert gas on the single crystal can be expected. However, considering the radiant heat from the single crystal is absorbed into a cooling chamber, it cannot be said that its cooling capacity is high.
Thereupon, as a method for solving the problems of the screen and the gas flow-guide cylinder and for efficiently cooling, there is proposed a method of arranging a water-cooled cooling cylinder around the crystal (See International Publication Pamphlet WO01/57293). In this method, an outside of the cooling cylinder is protected by a cooling-cylinder-protection material such as a protection cover made of graphite etc., and thereby the heat of the single crystal can be efficiently removed from the inside of the cooling cylinder. However, since the cooling cylinder does not extend to near the melt surface for safety, the cooling effect on the single crystal is somewhat low in a portion before reaching the cooling cylinder.
Moreover, there is disclosed a method of extending a graphite member etc. fitted into the cooling cylinder in Japanese Patent Laid-open (Kokai) No. H6-199590. However, this method cannot exert a sufficient cooling effect since the cooling cylinder and the extending graphite member are exposed to the heat from the outside and besides contact between the cooling cylinder and the graphite member is difficult. Consequently, the heat cannot be efficiently conducted from the graphite member to the cooling cylinder.